We don't know if Spock's definition would have covered something like the Miranda class, but if he can merely state a "configuration" rather than exact identity, then it's likely that the definition would at the very least cover a wide range of two-hulled two-nacellers in the appropriate size.
(Miranda Class) is referred to as a "starship" in Chekov's log entry on approach to Ceti Alpha V.
Again, the entire "Constitution Class" business is entirely the result of non-canonical but popular fan fiction having successfully brainwashed late Star Trek productions at the expense of the canonical producers' ("Starship Class") and creator's ("Enterprise Class") intentions.
Each time I see the bridge dedication plaque of the USS Defiant
in "In A Mirror Darkly" (ENT) I don't know whether I should laugh or cry. It says "Constitution Class" while the same spot on the Enterprise's
bridge dedication plaque is crystal clear: "Starship Class".